The invention relates to a method and computer program for the dimensional acquisition of an object required for reverse engineering or inspection tasks, which invention provides unskilled operators with an indication of acquisition quality the during the acquisition process. The inspection of physical objects usually comprises two different tasks: the acquisition of a numerical representation of an object, and the inspection of this numerical representation.
Dimensional acquisition (i.e. measurement) makes use of one or more dimensional measuring devices that are able to capture individual points on the surface of the object. Such dimensional measuring devices are well known in the art and might be, for example, a co-ordinate measuring machine arm (CMM arm) equipped with a laser scanner. CMM arms are manufactured by, for example, Faro Tech Inc, Hexagon Metrology AB or Metris. Laser scanners are currently manufactured by, for example, Faro Tech Inc, Perceptron Inc, Kreon Technologies, and Metris. A CMM arm is sometimes called a localizer because it is used to provide a 6 dimensional location (i.e. position and orientation) of the laser scanner. Other kind of localizers also exist such as conventional co-ordinate measuring machine (CMM) manufactured, for instance, by DEA, Mitutoyo, Wenzel, Zeiss, LK; or optical CMMs using light-emitting diodes (LEDS) or optical markers. Examples of optical CMMs include the Metris K600, laser trackers, laser radars. Apparatus combining a localizer and a scanner also exists such as in the Handyscan product of Creaform Inc.
In a laser scanner, a laser plane intersects an object and the intersection is captured by an imager apparatus that employs a CCD camera. Within the imager apparatus, the intersection is digitized in a series of 2D points and given the position reported by the localizer, these 2D points are converted to 3D points representing points located on the surface of the object being acquired.
Other dimensional measurement devices can be used to acquire points on the surface of an object, such a tactile probe (hard probes of deflecting probes) that acquires one point at a time; scanning probes that acquire points continuously while the probe is moving over the surface. Devices projecting structured lights also exist that are able to acquire points on the surface of an object (e.g., GOM systems).
Computed tomography (CT) scanners or robot CMM arms are also measuring devices able to measure points on the surface of the object. A CT scanner is also able to measure points on the internal surfaces of the object.
The resulting acquisition comprises a point cloud (a set of 3D points) or any other alternative representation such as stereolithography (STL) meshes, surfaces or simply raw range images. In the case of CT scanners, the result of the acquisition may be 3D voxel data where each voxels contains information about the density of the object.
These measuring devices may either be considered as manual devices i.e. a user is required to manipulate the device across the object to acquire data; alternatively, they may be automatic i.e. the measuring device, based on a definition of a path, automatically passes over the object to acquire data without human intervention.
A computer aided design (CAD) model is a computer generated numerical representation of an object to be manufactured. It results from building the object within the confines of a software CAD application rather than using dimensional acquisition data directly. A CAD model can be a represented by a set of points (a point cloud), a set of polygons (such as an STL mesh) or one or more surfaces (Non-uniform rational B-spline (NURBS), B-Rep, analytical surfaces). The CAD model may also contain additional information that is critical for the manufacturing process of the actual object. Such information is usually manufacturing tolerances and can be defined using the Geometric Dimensioning and Tolerancing (GD&T) ASME Y14.5M-1994 standard. The CAD model may also contain information of features such as edges, circles, holes, slots, round slots. These features may be represented by polylines, analytical curves, NURBS curves, etc . . .
During the inspection tasks, specific characteristics of an object can be measured. For example, a user may want to verify the diameter of a circular hole or its position relative to another hole. He may also want to verify the circularity of the hole. Another example of inspection task consists in the verification of the thickness between two surfaces. The user may also simply measure the distances between representative points of the surface. When a CAD model is available for the inspection tasks, the point cloud may be compared entirely with the CAD model, features of the CAD model (nominal features) may be compared with features of the point cloud (actual features).
The process of acquisition and inspection may be performed according to two different workflows. In a first workflow, the acquisition and inspection tasks are decoupled. Here, a shop floor worker simply acquires points on the surface of the object in order to capture the geometry. When the acquisition is complete, the point cloud is transferred to the inspection specialist, usually a skilled worker, who will perform the inspection tasks. This workflow suffers from some drawbacks. Since the shop floor worker is not guided during the acquisition process, he may provide a point cloud that does not cover the entire CAD model. Also, some regions may not be acquired with insufficient accuracy or resolution. For example, by using a CMM arm equipped with laser scanner, the shop floor worker is responsible to move the scanner with the correct speed. If he is moving too fast, the resulting cloud will be undersampled and will miss small features (small holes, fillets, edges, . . . ). Therefore, during the subsequent inspection, some inspection tasks will not be possible or will provide results that are not reliable. The only remedy is to go back to the shop floor and perform additional measurements which cost time or may even be impossible if the object has already been displaced, shipped to another location. To avoid this problem, the shop floor worker usually tends to over-sample the object, which means that too many points are acquired. The resulting point clouds constitute very large files, containing redundant information. Both the acquisition tasks and the inspection tasks require, therefore, extra time, and provide unwieldy files.
In a second (alternative) workflow, the acquisition and inspection tasks are performed sequentially, in that a part of the object is acquired, the acquisition stopped and an inspection is performed on that part. The process of stop/start acquisition and inspection is repeated until the inspection of the object is complete. In an example of the procedure, the user is first required to measure points corresponding to a specific feature (e.g. a slot) on the object, then to stop acquisition in order to create this feature using the measured points. After acquiring several of these features, the user needs to stop acquisition again to proceed to an alignment task where the position of the objects referenced according to the reference system of the measuring machine is related to the reference system of the CAD model expressed in part coordinate system. In the subsequent steps, the user is guided for the acquisition of the remaining points needed for the complete inspection of the part, stopping between sessions of acquisition and inspection. An example of such workflow is described in e.g. US 2006/0274327 where the operator is guided through a series of acquisition and inspection tasks. In US 2006/0274327, a new acquisition may be automatically requested to the operator if the inspection has not succeeded.
The second workflow requires a skilled worker since he needs to understand the both the acquisition and inspection processes i.e. to acquire points with sufficient density and accuracy, and to then to perform feature detection, alignment, tolerances, etc as part of the inspection process. For example, some computer programs readily allow points to be continuously compared with a CAD model (assuming the worker has already aligned the measuring system with the CAD model). However, these programs give a false sense of safety since they provide information on what has been acquired but not on what is missing or on the quality of the acquired data.
In this workflow, the inspection software and acquisition software are identical to that of the first workflow, thus creating complexity since the user needs to master two complex software applications. Further, time is wasted because the user has to stop acquisition and switch back and forth between acquisition and inspection software application and continuously transfer the newly acquired 3D points. Finally, the user must be able to read and understand the CAD data, correctly identify on the object the features or the location where points need to be acquired.
The acquisition of points on the surface of 3D objects may also be used in the context of reverse engineering. Here, the acquisition is guided by the need to acquire geometric information on the surface of the object with sufficient density as to be able to create a global numerical model, e.g. by means of NURBS surfaces or STL mesh. Too few acquired points will create holes in the model which will result in a model of poor quality where details are missing. Too many points will slow down the reverse engineering tasks since most of the reverse engineering operations require a time proportional to the number of points. Such a model can be later used for rapid prototyping, or visualization.